1. Field of the Invention
The present invention relates to an optical fiber suitable as an optical transmission path and a method for making such an optical fiber.
2. Related Background Art
Conventionally, a dispersion managed fiber as an optical transmission path composed of plural fiber sections having different fiber characteristics at respective sections and can solve problems not solvable by an optical fiber composed of a single kind of section is disclosed in U.S. Pat. No. 5,894,537, for example. In this dispersion managed fiber, a dispersion managed transmission path is constituted of sections having a positive chromatic dispersion and sections having a negative chromatic dispersion, so that the deterioration in transmission characteristics due to the nonlinear optical interaction among optical signals having different wavelengths and the distortion of optical pulses due to total chromatic dispersion can be reduced simultaneously.
Among methods for making such a dispersion managed fiber, following two methods are provided, for example. The first is a method which changes the refractive index of the core region along the fiber axis. For example, the core region is doped with such materials that the refractive index of core region changes by exposure to ultraviolet radiation. The fiber is exposed to ultraviolet radiation after fiber drawing so as to obtain a desired refractive index. The second is a method which changes the diameter of the core region along the fiber axis.
However, both of the above-mentioned two methods have problems as follows. In the first method, usually, since the change in refractive index induced by exposure to ultraviolet radiation is approximately 10xe2x88x923 and hence is small, it is difficult to change the chromatic dispersion widely along the fiber axis. Accordingly, the absolute value of the local chromatic dispersion cannot be increased and hence, it is impossible to sufficiently suppress the nonlinear optical interaction among optical signals having different wavelengths. Further, it is also difficult to change the sign of the chromatic dispersion slope along the fiber axis so that the total chromatic dispersion slope increases. As a result, the wavelength bandwith sufficiently small total chromatic dispersion gets narrow and hence, the capacity of the transmission path becomes small.
Further, in the second method, it is difficult to have the cross-sectional distribution of refractive index change drastically along the fiber axis. To realize a negative chromatic dispersion slope, the refractive index distribution having a depressed portion, i. e., a refractive index distribution having, between the core region having a high refractive index and the outer cladding region having a low refractive index, an inner cladding region (the depressed portion) whose refractive index is lower than the outer cladding region is suitable. On the other hand, to realize a positive chromatic dispersion slope, the refractive index distribution having no depressed portion, i. e., the refractive index distribution where the refractive index takes the minimum in the outer cladding region is suitable. However, it is usually difficult to make a preform where a section has a refractive index distribution having a depressed portion and another section has a refractive index distribution having no depressed portion. Accordingly, the absolute value of total chromatic dispersion slope becomes large and the wavelength band with sufficiently small absolute value of total chromatic dispersion becomes narrow.
Further, as the change in the chromatic dispersion along the fiber axis becomes steeper, the fabrication of the fiber becomes more difficult. For example, when the preform diameter is 50 mm and the fiber diameter is 125 xcexcm, to realize a change in chromatic dispersion at a period of 640 m along the fiber axis, it is necessary to change the ratio of the core diameter to the cladding diameter in the preform at a period of 4 mm along the preform axis. Accordingly, in case of grinding the core preform, a minute processing technique becomes necessary, and in case of elongating the core preform, a highly position-selective heating technique becomes necessary. Further, the shorter the period of the change in the chromatic dispersion along the fiber axis, the number of the parts in the preform to be processed increases so that the fabrication becomes laborious.
Conventionally, there has been known a dispersion compensating fiber which has negative chromatic dispersion and negative chromatic dispersion slope to compensate for positive chromatic dispersion and positive chromatic dispersion slope as disclosed in U.S. Pat. No. 5,995,695. However, a dispersion compensating fiber having positive chromatic dispersion and negative chromatic dispersion slope has not been known and hence, it has been difficult to compensate for negative chromatic dispersion and positive chromatic dispersion slope. A dispersion managed fiber including sections having positive chromatic dispersion and negative chromatic dispersion slope and sections having negative chromatic dispersion and positive chromatic dispersion slope has not been known either. Accordingly, in the conventional dispersion managed fiber, locally-zero-dispersion wavelength, at which local chromatic dispersion becomes substantially zero, is present in the short wavelength side of the operating wavelength range. The band in the vicinity of this wavelength is not suitable for the wavelength division multiplexing transmission because of the deterioration of the transmission quality due to the four-wave mixing or the cross phase modulation and hence, the conventional dispersion managed fiber cannot expand its operating wavelength range to the short-wavelength side.
So-called microstructured optical fiber, which has a high degree of freedom in setting the local chromatic dispersion is disclosed in U.S. Pat. No. 5,802,236. This microstructured optical fiber has microstructures (usually voids) in a cladding region and it is possible to increase the effective refractive index difference between the core region and the cladding region. As a result, this optical fiber can realize large absolute value of the chromatic dispersion and small mode field diameter.
A method for manufacturing such a microstructured optical fiber is disclosed in U.S. Pat. No. 5,802,236, wherein tubes and a rod are bundled to form a preform from which a microstructured fiber is drawn. Another method of making a microstructured fiber is disclosed in the International Publication WO00/16141 wherein a plurality of rods of given shape are bundled to form a preform from which a microstructured fiber is drawn.
In a microstructured optical fiber, to obtain desired characteristics with respect to chromatic dispersion or mode field diameter, it is important to accurately control the area fraction of void in the fiber, which is the ratio of the area occupied by the void to the area of a given region in the fiber cross section.
However, in the conventional method in U.S. Pat. No. 5,802,236, it is difficult to accurately control the area fraction of void because of the gaps among the tubes. To prevent the gaps among tubes from remaining in the drawn fiber, it is necessary to raise the pressures in the voids of the tubes above those in the gaps. However, such an operation is difficult because it requires selective manipulation of pressure in the preform. On the other hand, when the fiber is drawn so that the gaps among tubes remain as the voids in the fiber, it becomes unnecessary to perform the selective manipulation of pressure. However, it is difficult to maintain close contact of tubes so that the structure in the cross section of the preform is tend to be disordered. The same problems arise also in the method disclosed in the above-mentioned International Publication WO00/16141.
Further, to enhance the strength of the drawn optical fiber and to prevent the fiber from breaking during fiber drawing, it is desirable to raise the drawing temperature. However, when the temperature of the preform rises, the viscosity of the preform is decreased and the voids are apt to be collapsed. Accordingly, increase in the pressure inside the voids is necessary to raise the drawing temperature. However, as mentioned above, since selective manipulation of pressure is difficult in the conventional technique which forms the preform with tubes and/or rods, increase in the pressure inside the voids also elevates that in the gaps, so that the drawn fiber tends to have unintentional voids corresponding to the gaps in the preform and the microstructure in the cross-section of the drawn fiber tends to be disordered. As a result, it is impracticable to enhance the strength of the fiber by raising the drawing temperature.
As described above, conventionally, it has been difficult to fabricate a microstructured optical fiber which realize optical properties like local chromatic dispersion and mode field diameter as desired. It is much more difficult to realize the dispersion managed optical fiber whose optical properties are deliberately varied along the fiber length.
The present invention has been made in view of the above and it is an object of the present invention to provide a dispersion managed fiber with small total chromatic dispersion slope and a dispersion managed fiber which enables expansion of the operating wavelength band to the short wavelength side, a dispersion compensating fiber which realizes compensation of negative chromatic dispersion and positive chromatic dispersion slope, and a method of making such optical fibers easily and securely.
That is, the optical fiber according to the present invention is an optical fiber composed of at least a section of the first kind having chromatic dispersion not less than a given positive value x and negative chromatic dispersion slope at a given wavelength, and at least a section of the second kind having chromatic dispersion not more than xe2x88x92x and positive chromatic dispersion slope at said given wavelength.
According to such a constitution, distortion of optical pulse due to total chromatic dispersion and deterioration in transmission quality due to the nonlinear optical phenomena can be suppressed over a wide wavelength range, which can be used for the operating wavelength band. Further, the wavelength band adjacent to the operating band in the short wavelength side can be made free of the locally-zero dispersion wavelength at which local chromatic dispersion substantially becomes zero. In the wavelength band in the vicinity of the locally-zero dispersion wavelength, deterioration of transmission quality due to the above-mentioned nonlinear optical phenomena is liable to occur and hence such a wavelength band is not suited for wavelength division multiplexing. However, according to the optical fiber of the present invention, since the wavelength band in the short wavelength side of the operating band can be made free of the locally-zero dispersion wavelength, it becomes possible to expand the operating wavelength band toward the short wavelength side to meet an increase in demand.
Such an optical fiber can be realized by, for example, an optical fiber composed of a core and a cladding which surrounds the core and has a mean refractive index lower than that of the core, at least one of the core and cladding includes regions spaced apart in cross section and made of sub mediums whose refractive indices are different from those of main mediums constituting the optical fiber, and at least one of the cross-sectional areas and refractive indices of the regions made of the sub mediums change along the fiber axis.
Here, the main mediums are mediums which can practicably constitute an optical fiber by themselves such as silica glass. An optical fiber has to contain at least and not more than one region made of the main medium. On the other hand, the sub mediums are present in regions surrounded by the main mediums may be mediums which can not practicably constitute an optical fiber by themselves, such as gas. Vacuum also can be employed as sub medium.
By changing at least one of the cross-sectional areas and the refractive indices of regions made of the sub mediums along the fiber axis, a large change in chromatic dispersion along the fiber axis can be realized so that large local chromatic dispersion is realized whereby the nonlinear optical interaction among optical signals having different wavelengths can be sufficiently suppressed. Further, it becomes easy to drastically change the cross-sectional distribution of refractive index along the fiber axis. Accordingly, an optical fiber where the cross-sectional distribution of refractive index has a depressed portion in some fiber sections and does not have a depressed portion in other fiber sections can be easily realized. As a result, change in the sign of chromatic dispersion slope along the fiber axis can be realized, so that total chromatic dispersion slope can be sufficiently made small. Accordingly, the wavelength band with small absolute value of total chromatic dispersion can be broadened, resulting in increased transmission capacity.
It is preferable that the chromatic dispersion at this given wavelength is larger than 1 ps/nm/km in the section of the first kind and smaller than xe2x88x921 ps/nm/km in the section of the second kind and the total length of the fiber sections whose absolute values of the chromatic dispersion are below 1 ps/nm/km is less than {fraction (1/10)} of the full length of the optical fiber.
In this manner, the lengths of respective fiber sections and the chromatic dispersion values are designed so that the absolute value of local chromatic dispersion becomes large and the absolute value of total chromatic dispersion becomes small. As a result, an optical fiber where the deterioration of transmission quality due to the nonlinear optical phenomena among optical signals having different wavelengths and the distortion of optical pulses due to total chromatic dispersion are both reduced can be realized. Such an optical fiber can be suitably used as a transmission path for a large capacity optical communication.
Here, it is preferable that sections which do not contain sub mediums are spaced apart along the fiber axis. Such an optical fiber can be cleaved at the section which does not contain sub mediums in cross section and spliced to another optical fiber by fusion. Splicing is difficult in the conventional microstructured optical fiber because the microstructures make it difficult to observe the core for alignment and because the microstructures tend to be deformed and/or collapsed due to fusion resulting in weakened optical confinement and increased optical attenuation. On the other hand, in the present fiber, the problems of the deformation or collapse of microstructures due to fusion and difficulty in observing the core do not arise, so that fusion splice can be easily performed and optical attenuation at the splice can be made small.
It is preferable that at least one of the cross-sectional areas and the refractive indices of the regions made of the sub mediums change at a given period along the fiber axis and the other are uniform or change at same period along the fiber axis. According to such a constitution, an optical fiber having the local chromatic dispersion periodically changed along the fiber axis can be realized. In such an optical fiber, even when the fiber length is changed by an integer times of the period of the dispersion change, the wavelength at which total chromatic dispersion becomes zero is not changed. Since the change of the chromatic dispersion characteristics of the transmission path due to the change of the length of the transmission path can be reduced, it becomes easy to change the length of the transmission path without affecting the transmission quality. It is preferable that this period exceeds 1 m.
It is preferable that the optical fiber has at least a transition section of a given length or more where at least one of the cross-sectional areas and the refractive indices of the regions made of the sub mediums change continuously along the fiber axis and the other are uniform or change continuously along the fiber axis. According to such a constitution, change in the cross-sectional area and the refractive index distribution along the fiber axis can be made sufficiently mild so that the loss due to the mode coupling at the transition sections can be reduced. As a result, the transmission loss across the whole fiber can be reduced.
The optical fiber may be constituted such that the main medium is silica glass and the sub medium is air. According to such a constitution, by adjusting the furnace temperature or pressure in voids during fiber drawing, the cross-sectional areas of the voids can be easily changed along the fiber axis. Since the relative index difference between silica glass and air is as large as approximately 35%, it is possible to drastically change the chromatic dispersion by changing the cross-sectional areas of the voids. As a result, the change of the chromatic dispersion characteristics along the fiber axis can be made more drastic than that in the conventional dispersion managed fiber. Further, since the transparency of silica glass and air is high, the transmission loss of the optical fiber can be suppressed.
Further, the optical fiber may have the chromatic dispersion not less than a positive value x and the negative chromatic dispersion slope at a given wavelength. According to such a constitution, it becomes possible to compensate for negative chromatic dispersion and positive chromatic dispersion.
Such an optical fiber can be realized by an optical fiber composed of a core and a cladding surrounding the core and having a lower mean refractive index than that of the core, wherein at least one of the core and the cladding has regions spaced apart in cross section and made of sub mediums whose refractive indices are different from those of main mediums constituting the core and the cladding.
According to such a constitution, large waveguide dispersion can be realized so that an optical fiber having positive chromatic dispersion and negative chromatic dispersion slope can be realized. Further, the absolute values of chromatic dispersion and chromatic dispersion slope can be made large, so that the fiber length required for compensation of dispersion and dispersion slope can be shortened.
The optical fiber of the present invention can be made by a method for making an optical fiber having voids extending along the fiber axis, comprising the steps of preparing the preform having a plurality of voids whose cross-sectional areas are uniform along its axis, and drawing the optical fiber from this preform, wherein a means to measure the area fraction of voids in the drawn optical fiber, a means to adjust the pressure in the voids of the preform and a means to feedback the measured area fraction of voids to adjusting means are included.
The cross-sectional areas of voids in the drawn optical fiber depend on the pressure in the voids during fiber drawing. Accordingly, by adjusting the pressure in voids during the fiber drawing, the cross-sectional areas of the voids in the drawn optical fiber can be varied as desired. Further, since it is unnecessary to change the cross-sectional structure of the preform along its axis, the optical fiber can be easily fabricated compared to the conventional fabrication technique. Further, since the pressure in voids can be changed rapidly, the structure in which the cross-sectional distribution of the refractive index of the fiber changes steeply along the fiber axis can be easily fabricated. As a result, the method is suitable as a method for manufacturing the above-mentioned optical fiber according to the present invention. Further, since the area fraction of the voids in the drawn optical fiber is measured and the result of the measurement is feedbacked to the pressure adjusting means, the fluctuation in the structure of the drawn optical fiber along its axis due to the fluctuation in the structure of the preform along its axis and the temporal fluctuation in the fiber drawing environment can be suppressed, whereby an optical fiber with desired optical characteristics can be fabricated with high yields. The area fraction of the voids is defined in the cross-section of a preform or a fiber as the ratio of the total area of the voids to the area of the cross section.
Alternatively, the method of making an optical fiber according to the present invention is a method of making an optical fiber which contains a plurality of regions made of sub mediums whose refractive indices differ from those of main mediums constituting the optical fiber comprising the steps of preparing a preform having a plurality of regions made of sub mediums whose cross-sectional areas are constant along the preform axis, and drawing the optical fiber from this preform, wherein a means to adjust the heating condition through varying at least one of the temperature of the drawing furnace for heating the preform and the time length for the fiber to pass the drawing furnace is included.
Change in the cross-sectional areas of the sub-medium regions during fiber drawing depends on the temperature in the drawing furnace and the time length to heat the preform. By changing at least either of the temperature in the drawing furnace and the time length to heat the preform, it is possible to change the cross-sectional areas of the sub-mediums in the drawn optical fiber along the fiber axis. As a result, the above-mentioned optical fiber according to the present invention can be favorably fabricated.
Here, it is desirable to measure the area fraction of voids or sub-medium regions in the drawn optical fiber, and feedback control the temperature in the drawing furnace and/or the time for heating the preform with the area fraction of voids or sub-medium regions thus measured. According to such an operation, the fluctuation in the structure of the drawn optical fiber along its axis due to the fluctuation in the structure of the preform along its axis and temporal fluctuation in the fiber drawing environment can be suppressed, whereby an optical fiber with desired optical characteristics can be fabricated with high yields.
For obtaining the area fraction of voids or sub-medium regions in the drawn optical fiber, the following means can be employed. In the first means, the speed at which the preform is supplied, the speed at which the fiber is drawn and the fiber diameter during fiber drawing are measured, and the area fraction of voids (or sub-medium regions) in the drawn optical fiber is calculated from these measured values, the preform diameter and the area fraction of voids (or sub-medium regions) in the preform, wherein the latter two quantities are measured before fiber drawing. Since the glass volume of the fiber drawn during a given period is equal to the glass volume of the preform supplied during the same period, the area fraction of the voids (or sub-medium regions) in the drawn fiber can be obtained from measurement of the above-mentioned quantities. In the second means, the speed at which the fiber is drawn, the fiber diameter, the drawing tension and the temperature in drawing furnace during fiber drawing are measured, and the area fraction of voids (or sub-medium regions) in the drawn optical fiber is calculated from these measured values. Since the drawing tension is related to the area fraction of voids (or sub-medium regions), the drawing speed, and the furnace temperature, the area fraction is obtained from measurement of the above-mentioned quantities. Since the area fraction of voids or the sub-medium regions can be grasped during the fiber drawing with above-mentioned techniques, an optical fiber with desired optical characteristics can be fabricated with high yields by them to the fiber drawing conditions.
Further, it is preferable that the method according to the present invention further comprises the preprocessing step of making a preform in a single piece, boring three or more voids in the preform along its axis, and cleaning the surfaces of the preform at the voids, and the drawing step includes a means to prevent contaminants from intruding into these voids.
The preform fabricated in this manner, different from those made by the conventional method of bundling tubes and/or rods, have no voids formed by the gaps among the tubes and/or the rods. Accordingly, it becomes easy to control the area fraction of voids in the drawn fiber to the desired amount, whereby an optical fiber with desired optical characteristics can be fabricated with high yields. Further, since the cleaning the wall surfaces of the voids is facilitated, the optical fiber with low transmission loss can be fabricated. And since the preform is formed in a single piece, the reproducibility of the fabrication can be also enhanced.
For boring the voids in the preform, it is desirable to insert boring appliances into the preform at a temperature above the glass softening point, and pull out the boring appliances from the preform immediately before or after lowering the temperature of the preform. Since the viscosity of the preform is low when it is bored, the energy required for boring can be reduced and thin and deep voids with uniform diameter can be easily formed so that the yields of the obtained optical fiber can be enhanced.
Alternatively, the method of making an optical fiber according to the present invention is a method of making an optical fiber which contains a plurality of regions made of sub mediums having refractive indices different from those of main mediums constituting the core and the cladding, comprising steps of injecting a medium whose refractive index is changeable on exposure to radiation into given regions of the optical fiber, and varying the refractive index of said injected medium along the fiber axis by exposing the fiber to radiation.
Here, since the desired refractive index profile can be obtained by injecting the medium having a large sensitivity of the refractive index change on exposure to radiation such as electromagnetic wave and electron ray, and thereafter exposing the optical fiber to radiation, compared with a case in which an optical fiber is constituted only by silica glass, the change in the cross-sectional distribution of refractive index along the fiber axis can be increased. Further, the change of the refractive index is performed not on the preform but on the fiber so that the optical fiber can be fabricated easily even with a structure where the change in the cross-sectional distribution of refractive index along the fiber axis is steep.
Alternatively, the method of making an optical fiber according to the present invention is a method for making an optical fiber having a plurality of voids, comprising the step of closing the voids by heating and fusing the drawn optical fiber selectively at a plurality of portions spaced apart along the fiber axis.
According to such a constitution, the cross-sectional areas of the voids along the fiber axis can be changed after drawing the optical fiber. According to the present invention, it is unnecessary to change the fiber drawing environment during fiber drawing to produce the change of the cross-sectional areas of the voids along the fiber axis and hence, the optical fiber can be easily fabricated. Further, the change of the refractive index is performed not on the preform but on the fiber so that the optical fiber can be fabricated easily even an optical fiber whose cross-sectional distribution of the refractive index distribution changes steeply along the fiber axis can be easily fabricated.